Gas flow control apparatus for an anaesthesia system

ABSTRACT

An apparatus for controlling the flow of gas to and from a patient in an anaesthesia system has first and second flow passages arranged in parallel between the patient and a reservoir bag. Selector valves allow multiple anaesthesia configurations involving no rebreathing or considerable rebreathing. A selector valve can be operated to connect the reservoir bag to either or both flow passages. A one way valve in the first passage, which communicates with a gas supply inlet, can be operated to prevent the back flow of gas from the patient into the passage. A one way valve in the second passage can selectively prevent the flow of gas in the second passage to the patient. The second passage has a selectively bypassable carbon dioxide absorbing fitter and a spill valve. The apparatus allows reconfiguration without disconnecting and reconnecting hoses.

This invention relates to apparatus for controlling the flow of gas toand from a patient in an anaesthesia system, and relates in particularto such apparatus whereby any one of several gas supply circuits may beselected without the need to change the connecting lines extendingbetween the apparatus and the patient.

In present day anaesthetic practice there are many different systems forthe supply of anaesthetic gases to and from a patient, each havingparticular characteristics relating to the gas flow to the patient froman anaesthesia machine and to the expiratory flow from the patient. Incertain of these systems the patient may either breathe spontaneously,or, alternatively, mechanical ventilation may be provided, while othersystems may only be used satisfactorily with one or the other mode ofbreathing. In addition, some systems cause the patient to rebreathe aportion of his expired gases, while in other systems there is little orno rebreathing and the only the fresh gas supplied by the anaesthesiamachine enters the patient's lungs.

Commonly used gas supply systems which are familiar to those skilled inthe art include the Mapleson A and Mapleson D systems. The former whileallowing little to no rebreathing is generally suitable only for apatient who breathes spontaneously, and the latter, which does allowrebreathing, may be used with a spontaneously breathing patient or wherea mechanical lung ventilator is required. Other systems which may beapplicable depending on particular clinical conditions, include theso-called circle system where expired gases are passed through a carbondioxide absorbing means before being returned to the patient.

In the conventional anaesthesia apparatus, when it is desired to switchfrom one gas supply system to another, it has generally been necessaryto disconnect and reconnect hoses or pipes in the apparatus inaccordance with the particular gas supply system required. This processis time consuming and inconvenient to the physician, and considerablecare and attention must be given to ensure that the correct connectionsare made so that undesirable and potentially dangerous configurationsare avoided.

In accordance with the invention there is provided apparatus forcontrolling the flow of gas to or from a patient in an anaesthesiasystem. The apparatus has a reservoir bag, and a selector valve meansoperable to connect the reservoir bag to either or both of first andsecond flow passages. The passages are arranged in parallel and adaptedto be connected to a patient at on end remote from the selector valvemeans. The first passage has a first one-way valve means selectivelyoperable to prevent the back-flow of gas from the patient into thepassage. The first passage communicates with an inlet for the supply offresh gas. A second flow passage has a second one-way valve meansselectively operable to prevent the flow of gas from the passage to thepatient and a selectively bypassable carbon dioxide absorbing means,there being a spill valve arranged to release gases from the second flowpassage. Means are provided to control the conditions of the selectorvalve means and first and second one-way valve means whereby desiredmodes of operation of the apparatus may be selected.

The fresh gas supplied to the first flow passage via the inlet willdepend on clinical conditions and may, for example, consist of a mixtureof oxygen, nitrous oxide and anaesthetic vapour. As in conventionalanaesthesia systems a number of different gases may be used.

In accordance with the invention, various modes of operation,corresponding to a number of different traditional gas supply systems,may be selected without the need manually to disconnect and reconnecthoses forming part of the apparatus or those extending between theapparatus and the patient. At the same time, the arrangement may be suchthat undesirable and potentially dangerous modes of operation areavoided. This is very important if safety standards are to bemaintained.

In a first mode of operation, the selector valve means is actuated sothat the reservoir bag is connected to the first flow passage while theflow of gas between the second flow passage and the bag is prevented. Inthis mode both one-way valves are retained in an inoperative conditionin which they allow flow in each direction. and The carbon dioxideabsorbing means in the second flow passage, for example a filtercontaining soda-lime in granular form, is bypassed. Those skilled in theart will recognize this mode of operation of the apparatus as beingequivalent to a Mapleson A gas supply system, which is particularlysuited for use with a patient who breathes spontaneously.

A second mode of operation corresponds to a Mapleson D system. In thismode the selector valve means is actuated such that the reservoir bag isconnected to the second flow passage and is shut off from the first.This mode promotes a considerable degree of rebreathing, since the freshgas is supplied via the first passage which is shut off from thereservoir bag and thus forms a low compliance line.

The third mode of operation corresponds to a circle system which againpromotes a considerable degree of rebreathing. In this mode, theselector valve means is actuated such that the reservoir bag isconnected to both the first and the second flow passages. In addition,the one-way valve means are operated such that gas may only be suppliedto the patient via the first flow passage, and expired gases may onlyleave the patient via the second passage. In this mode gases areeffectively recirculated to and from the patient via the first andsecond passages, compliance being provided by the ventilation bag.Generally, in this mode the soda-lime filter will not be bypassed sincecarbon dioxide needs to be removed from the expired gases before theseare recirculated to the patient.

A fourth mode of operation allows no rebreathing whatsoever. In thismode, the one-way valve means are again operated, and the selector valvemeans is actuated such that the ventilator bag is connected to the firstflow passage and shut off from the second.

In a final mode, the fresh gas supply may be isolated such that a commongas outlet configuration is provided to which any conventional type ofanaesthetic breathing system may be attached if it is not desired to useone of the preferred modes discussed above. In such a configuration theone-way valve means remain inoperative.

In all the above modes of operation, excess gas may be vented from theapparatus via the spill valve communicating with the second flow passageas the patient exhales. As is conventional, the spill valve couldcomprise a non-return valve adapted to prevent air entering the system.

With the exception of the Mapleson A and common gas outletconfigurations, each of the above modes of operation are suitable foruse in controlled, mechanical ventilation as well as for use with areservoir bag where a patient breathes spontaneously. Thus, in apreferred embodiment, the apparatus also has a mechanical ventilator andswitching valve means which can be actuated to connect either themechanical ventilator or the reservoir bag to the selector valve means.Thus, in this embodiment, either an automatic mode, wherein themechanical ventilator is actuated and is connected to the selector valvemeans or a spontaneous breathing mode wherein the reservoir bag isconnected to the selector valve means, may be selected. In the lattermode, as with conventional systems, breathing may be promoted manuallyby the physician squeezing the reservoir bag, if necessary.

Where mechanical or hand controlled ventilation is used, it is importantthat during inflation of the patient's lungs gases cannot escape fromthe apparatus via the one-way spill valve. Therefore, it is meanspreferable to provide for preventing opening of the spill valve insynchronism with inhalation during mechanical or hand controlledventilation.

In a preferred embodiment, the mechanical ventilator is a compressiblebellows mounted within a rigid, which chamber can be pressurized andopened to atmosphere in a cyclical manner by, for example, anelectronically controlled valve. The chamber is pressurized by a drivegas and, when pressurized, the bellows are compressed, forcing theanaesthetic gases into the lungs of the patient. When the chamber isopened to atmosphere, the relaxation of the patient's chest and lungswill cause the gas to flow from the lungs back into the bellows. In thisembodiment, the sealing of the spill valve during inflation may beachieved by means of a pressure sensitive closure member of the spillvalve in flow communication with the drive gas inlet of the mechanicalventilator. Similarly, where hand controlled ventilation is used by thephysician, the closure member is in flow communication with the outletfrom the reservoir bag, such that, when the bag is compressed by thephysician, the resulting increase in pressure causes the spill valve tobe closed.

The structure of the various valve means and the nature of the means forcontrolling the conditions of various valve means in accordance with theinvention may vary. In one embodiment the apparatus is monitored andcontrolled by a suitable electronic control means and the various valvemeans are individually actuable in response to control signals from thecontrol means. The control means is programmed to select the desiredoperational configuration discussed above and to avoid unwanted,potentially dangerous modes. In such an embodiment, the valves mayconveniently comprise solenoid controlled valves.

In an alternative embodiment, a mechanical actuating means may beprovided for certain of the valve means. In one such embodiment, theselector valve means comprises cam operated poppet valves, the valveclosure members of which are operated by cams carried by a rotatableshaft. Rotation of the shaft thus is effective to change the setting ofthe selector valve means. In such an arrangement, a valve arranged tocontrol whether or not the carbon dioxide absorbing means is bypassedmay likewise conveniently comprise a poppet valve operable by a futhercam carried by the rotatable shaft. The shaft may be rotated eithermanually or by an electric motor responsive, to electronic control meansof the apparatus. Preferably, suitable sensing means are provided, suchthat the control means may monitor the angular position of the shaft andthus the condition of the valve means.

In a preferred embodiment, the one-way valve means in the first andsecond flow passages comprise gravity operated disc valves. In oneembodiment, such valves are rendered inoperable simply by beingselectively bypassed when not required. In a preferred embodiment,however, each valve may be made operable or inoperable by the loweringor raising of a pin or rod arranged to extend through the valve seat. Inthe raised condition the valve closure member is prevented from seatingthat gases can flow either way through the valve. Such valves mayconveniently be pneumatically operable. In a preferred embodiment theactuating pressure signals are controlled by a further cam actuatedpoppet valve mounted on a common unit with the preferred selector valveand carbon dioxide absorber bypass valve discussed above.

It is desirable that the selector valve means and the one way valvemeans be constructed so that in the event of a power failure theapparatus will remain in a selected mode. It will be appreciated thatthis will be so in the case of the preferred cam actuated poppet valvesdiscussed above. By contrast, in those embodiments comprising amechanical ventilator, the switching valve which selects either themechanical ventilator or the ventilator bag is preferably biased sothat, in the event of either power failure or gas supply failure, thevalve resumes a position wherein the bag is connected to the patient topermit spontaneous breathing or manual ventilation. In a preferred suchembodiment, the switching valve is operated by gas pressure, under thecontrol of an electrically operable solenoid valve, so that both theelectricity and gas supplies must be provided to switch the valve to themechanical ventilation mode.

In a preferred embodiment, electronic control means for the apparatusforms part of a central microprocessor adapted automatically to controland monitor other aspects of an anaesthesia system. As mentioned above,means may be provided to sense and provide a signal indicative of thecondition of the various valve means, such that the control means maymonitor whether or not the apparatus is functioning correctly and hasadopted the selected mode. A suitable alarm signal may be given in theevent of a valve failure.

Certain embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of one embodiment of apparatus inaccordance with the invention;

FIG. 2 is a schematic view of the mechanical ventilator drive circuit;

FIG. 3 is a block diagram of the control circuitry;

FIGS. 4, 5 and 6 illustrate the apparatus shown in FIG. 1 in differentmodes of operation;

FIG. 7 is a schematic diagram of an alternative embodiment;

FIG. 8 is a side elevation of the valve unit for the embodiment shownschematically in FIG. 7;

FIG. 9 is an end elevation of the unit shown in FIG. 8;

FIG. 10 is a cross section taken along line X--X in FIG. 8;

FIG. 11 is a cross-sectional view on an enlarged scale of thepneumatically operated switching valve incorporated in the unit shown inFIGS. 8 to 10;

FIG. 12 is a schematic cross-sectional view of a one way valve;

FIGS. 13a to 13d illustrate schematically the operational configurationsof the valve unit; and

FIGS. 14 and 15 illustrate the apparatus shown in FIG. 7 in differentmodes of operation.

Referring firstly to FIG. 1, the apparatus for controlling the supply ofgases to and from a patient in an anaesthesia system comprises areservoir bag 1, a selector valve 2 and first and second flow passages3,4 terminating at patient connection ports 5,6. The ports 5,6 may beconnected by flexible plastic or rubber hoses of suitable length to aY-piece (not shown), the third limb of the Y forming the connection portto the lungs of the patient.

The first passage 3 includes a fresh gas inlet 7 which in use isconnected to a source of anaesthetic gases, for example, a mixture ofnitrous oxide, oxygen and anaesthetic vapour. In addition, a line 8 isprovided which is connected to a source of oxygen. The oxygen may beused initially to flush the apparatus, or alternatively may be used inan emergency to resuscitate a patient. The second flow passage 4includes a soda-lime filter 9 which may selectively be bypasseddepending upon the setting of a valve 10. In addition, both passages 3,4 include one-way valves 11,12 adjacent the ports 5,6. The one-wayvalves may or may not be operative depending on the condition of theadjacent valves 13,14 as will be described in more detail below.

The apparatus additionally comprises a mechanical ventilator 15 whichmay be connected to the selector valve 2 in preference to the reservoirbag 1 by means of a switching valve 16. The selector valve 2 is actuableto connect either or both of the flow passages 3,4 to the switchingvalve and thus to the bag 1 or to the ventilator 15.

A spill valve 17 communicates with the second flow passage 4 andincludes a pressure sensitive sealing member 18 which may be connectedby means of a valve 19 either to the outlet from the bag 1 or to thedrive gas circuit for the mechanical ventilator 15.

As shown schematically in FIG. 2, the mechanical ventilator 15 includescompressable bellows 20 mounted in a rigid chamber 21. The chamber 21can be pressurized and opened to atmosphere in a cyclical manner bymeans of electronically controlled valves 22,23, the valve 22controlling the inlet of the pressurizing drive gas and the valve 23controlling the drive gas supply to a pressure sensitive release valve24.

A stepper motor 25 controls the rate of flow of the drive gas into thechamber 21. The motor 25 and the valves 22,23 are responsive to controlmeans of the apparatus, described in more detail below, such that whenmechanical ventilation is used, the breathing frequency may be selectedand accurately regulated.

Turning now to FIG. 3, electronic control means for the apparatus shownin FIG. 1 are schematically illustrated. A switch panel 25 enables theoperator to select one of a number of possible modes of operation, thesignals from the switch panel being input into a microprocessor 27 via abus 26. The microprocessor is preprogrammed such that, in response to aparticular mode selected by the operator, output signals are applied todrivers 28,29,30. These in turn actuate the various valves of theapparatus in accordance with the selected mode, as will be explained inmore detail below.

The valves 2, 10, 13, 14, 16 are illustrated schematically only in FIG.1 for the purposes of describing the various modes of operation of thesystem. It will be appreciated that the valves should be so constructedas to be responsive to the control signals from the microprocessor toprovide the required switching between the different operationalconfiguration discussed below. Such valves may conveniently be solenoidcontrolled. Furthermore, the valves of the apparatus preferably includesensing means (not shown), for example Hall Effect sensors, adapted toprovide a signal indicative of the condition of the respective valves,so that the microprocessor may monitor the state of each valve andprovide an emergency signal in the event of valve failure.

Referring now to FIG. 1, the various modes of operation of the systemwill be described.

In FIG. 1, the valves are set by the microprocessor such that theconfiguration corresponds to a Mapleson A gas supply system. Thus, theselector valve 2 and the switching valve 16 are set such that thereservoir bag 1 is connected to the flow passage 3 and is shut off fromthe flow passage 4. The valves 13,14 are set such that the one-wayvalves 11,12 are effectively bypassed, and the valve 10 is effective tobypass the soda-lime filter. In this mode of operation, which isparticularly suited for use with a patient who breathes spontaneously,the reservoir bag stores fresh gas, which enters at a constant flow ratevia the inlet 7 during the expiratory phase. During inspiration thepatient inhales gas from the bag at a rate much greater than thecontinuous fresh gas flow rate. Excess expired gas is vented from thesystem via the spill valve 17. In this mode, there is limitedrebreathing, since the fresh gas entering via the inlet 7 swamps theexhaled gases during filling of the bag 1, and because the volume of theexpiratory limb (in this mode the flow passage 4) is limited. Flowmeters 40 are provided in both passages 3,4 so that the microprocessormay monitor the volume of gas passing to and from the patient.

FIG. 4 illustrates a second mode of operation of the apparatusequivalent to a Mapleson D gas supply system. In this mode of operation,the control means actuates the selector valve 2 such that the portleading to the first flow passage 3 is closed off, and that leading tothe second flow passage is opened connecting the bag 1 to the secondflow passage 4. Thus, the bag 1 has effectively been transferred to theexpiratory limb, and fresh gas is delivered to the patient through a lowcompliance, small volume inspiratory limb (the closed flow passage 3).Lung ventilation is produced by a displacement of exhaled gas to andfrom the bag 1, with an escape of gas from the spill valve 17 duringexhalation being equal in volume to the fresh gas flow. As such, in thismode of operation there is considerable rebreathing by the patient ofexhaled gas. In the mode of operation shown in FIG. 4, the physician maybe required manually to ventilate the patient by squeezing the bag 1, soas to force gas into the patient's lungs. In this event, a manual modeswitch is depressed on the control panel and the microprocessor actuatesthe valve 19, such that the spill valve closure member 18 is in flowcommunication with the bag outlet. In this way, as the physiciansqueezes the bag the resulting increase in the pressure causes the spillvalve to be shut so that gases cannot escape via the spill valve duringinflation of the lungs.

The mode of operation illustrated in FIG. 4 is also suitable forautomatic ventilation by means of the mechanical ventilator 15. If thismode is desired, the appropriate switch is depressed on the controlpanel and the microprocessor then causes the switching valve 16 tochange state such that the port leading to the ventilator is opened andthe port leading the to the bag is shut. In this mode, themicroprocessor also causes the valve 19 to switch over so that thepressure sensitive closure member of the spill valve is in flowcommunication with the drive gas inlet of the mechanical ventilator.Thus, when the drive gas is forced into the ventilation chamber so as tocompress the bellows and inflate the patient's lungs, the increase inpressure causes the spill valve closure member 18 to remain in theclosed condition. In the automatic mode, the driver 29 in response tothe signals from the microprocessor is also effective to actuate thedrive gas control valves 22,23 and the stepper motor 25 so that thefrequency of the ventilator is accurately regulated.

Referring now to FIG. 5, a third mode of operation is illustratedcorresponding to a circle system. In this mode of operation, the valves13,14 are switched over so that the flow of gas is directed through theone-way valves 11,12. Thus, in this mode gases can only enter thepatient's lungs via the flow passage 3 and can only leave the patientvia the flow passage 4. In this mode of operation the selector valve 2is actuated such that all the ports thereof are open and the bag 1 isthus connected to both limbs of the apparatus. This system permitsconsiderable rebreathing, since gases are recirculated to and from thepatient. Accordingly the valve 10 is generally switched over in thismode, so that the gases are passed through the soda-lime filter wherebycarbon dioxide will be absorbed before gas is rebreathed. With thisconfiguration, the fresh gas flow may be related to metabolicrequirements rather than to the required volume of the respired gas. Itmay be desirable to operate the system with the soda-lime filterbypassed, in which case a higher fresh gas flow is required. As in themode of operation illustrated in FIG. 4, the circle system mayalternatively be used for mechanical ventilation. In this case theautomatic mode switch is depressed such that the microprocessorappropriately actuates the valves 16 and 19 as discussed above.

Finally, FIG. 6 illustrates a fourth mode of operation which permits norebreathing whatsoever. In this mode, the bag 1, or the ventilator 15,is again connected to the first passage 3, the selector valve 2 beingshut off from the passage 4. As in the FIG. 7 mode, the valves 13,14 areset such that the one-way valves 11' and 12' are operative. It will beseen that in this mode with no flow communication between the passages3,4, the one-way valves are effective to prevent rebreathing. This modeof operation is particularly suited to the use of air rather thanoxygen-rich mixtures and can be used either for spontaneous ormechanical ventilation.

It will be seen that the apparatus in accordance with the inventinenables different modes of operation to be selected quickly without theneed to change hose connections. The microprocessor is programmed suchthat only desired combinations of the valve settings may be selectedavoiding potentially dangerous configurations.

It will be appreciated that the specific arrangement of valves describedby way of example above may be varied. For example, instead of a singleselector valve 2, separate valves may be provided for each flow passage,which may be actuable independently of one another in order to connectthe the switching valve to either or both flow passages 3, 4.Furthermore, in an alternative arrangement, the valves 13, 14, which areeffective to bypass the one-way valves 11, 12 may be omitted. Insteadone-way valves may be provided adjacent to ports 5, 6, the operation ofeach being directly controlled by the microprocessor.

Turning now to FIG. 7 an alternative embodiment of the apparatus isillustrated wherein the selector valve 2, the switching valve 16 and thevalve 10, for controlling whether or not the carbon dioxide filter isbypassed, are combined into a single unit 50, as is a valve 51, whichcontrols whether or not the one-way valves 11,12 are operable, as willbe described in greater detail below. The remaining components of theapparatus shown in FIG. 7 correspond generally to those shown in theFIG. 1 embodiment, and like reference numerals have been used toillustrate like components. Furthermore, the mechanical ventilator drivecircuit illustrated in FIG. 7 corresponds to that shown in FIG. 2. Itwill be seen that, like the FIG. 2 arrangement, a solenoid controlledvalve 19 is provided to communicate either the bag or the mechanicalventilator with the spill valve 18 to ensure that the spill valve isclosed as gas is forced to the patient during controlled ventilation.

Referring to FIG. 12, the details of the one-way valves 11', 12'provided in the flow passages 3,4 of the FIG. 7 embodiment areillustrated. It will be seen that each valve includes a rigid disc 53which is guided with minimum friction so that it engages a horizontalvalve seat 54 under the influence of gravity in the operationalcondition of the valve. In this condition, the passage of gas in theforward direction is permitted once the pressure is sufficient toovercome the weight of the disc, but reverse flow is prevented as gaspressure then increases the closing force on the disc. Means forrendering the one-way valve inoperable in are provided the form of a rod55 guided through a sealed bearing 56 in the base of the valve andjoined to a pressure responsive element 57 such as a spring loadedbellows. A sealed pressure vessel 59 encloses the element 57 so that apressure applied to an inlet port 60 will compress the element 57 andraise the rod 55. As shown in broken lines, in the raised position ofthe rod 55, the disc 53 is unable to close on the seat 54, and thereforeflow of gas through the valve in both directions is permitted. Pressuresignals are applied to the inlets 60 of the valves 11', 12' via a line61 (FIG. 7). Line 61 is connected to the supply of drive gas for themechanical ventilator via a valve 51 forming part of the unit 50described in more detail below.

Turning to FIGS. 8 to 10, the details of the valve unit 50 areillustrated. The unit 50 comprises a block 62 secured to a cover plate63 defining the valve chambers therebetween. The coverplate 63 includesa port 64 for connection to the second flow passage 4, a port 65 forconnection to the carbon dioxide absorber, and a port 66 for connectionto the first flow passage 3. A further port 67 is provided forconnection to the spill valve 18.

The selector valve 2 comprises two poppet valve assemblies 68 arrangedto be forced into sealing contact with respective valve seats 69 bymeans of cams 70 carried by a rotatable shaft 71. Similarly, valve 10compises a poppet valve assembly 68 likewise operable by means of a cam70 carried by shaft 71.

The unit 50 further comprises the actuating valve 51 (not shown indetail) for the one-way valves 11', 12'. Valve 51 is also actuable underthe action of a cam 70' carried by the shaft 71. The unit 50 also mountsthe switching valve 16. The valve 16 includes ports 75, 76 forconnection respectively to the mechanical ventilator and the ventilatingbag.

The details of the switching valve 16 are shown in FIG. 11, from whichit will be seen that the valve includes an axially moveable closuremember 77 mounted on a shaft 78 moveable between a first positionwherein inlet port 79 (FIG. 10) of the valve 16 communicates with theport 76 connected to the bag, and a second position wherein the inlet 79communicates with the port 75 connected to the mechanical ventilator.the valve 16 is actuated into the second position by the application ofa pneumatic signal on a diaphragm 80 engaging the upper end of the shaft78. The pneumatic signal may conveniently be supplied automatically uponswitching of the solenoid valve 19 into the condition wherein mechanicalventilation can occur. The valve 16 further comprises a biasing spring81 arranged to urge the closure member 77 into its first condition whenactuating pressure is removed from the diaphragm 80. In this way, in theevent of electrical power or gas supply failure, the valve 16 isreturned to the condition wherein the ventilator bag rather than themechanical ventilator is connected to the patient. This is importantfrom the safety point of view. The shaft 78 of the valve 16 is providedat one end with a magnet 82 arranged to operate a Hall Effect switchassociated with an end cap 83 whereby the microprocessor may monitor thecondition of the switching valve.

Referring once more to FIGS. 8 to 10, the shaft 71 carrying theactuating cams 70 can be rotated by means of a suitable motor (notshown) controlled by the microprocessor or alternatively can be adaptedto be rotated by hand. A magnet 85 is mounted on an arm extendingradially from one end of the cam shaft and is located adjacent a printedcircuit board 86 which carries a series of Hall Effect switches arrangedso that the rotational position of the cam shaft can be encodedelectrically and thus monitored by the microprocessor. (It will be notedthat the printed circuit board and magnet have been removed from FIG. 9which is partly broken away to illustrate the cam 70' which engages theactuating valve 51 for the one-way valves 11',12').

It will be seen that rotation of the cam shaft 71 is effective tocontrol the operation of valves, 2, 10, 51 and thus 11',12'simultaneously. The operational sequence of the valves is illustrated inFIGS. 13a to 13d. Referring first to FIG. 13d, in a first angularposition of the cam shaft 71 the valve 10 is open, while the selectorvalve 2 is set so that the inlet port 79 of the switching valve isconnected to the flow passage 3. Furthermore, the actuating valve 51 isactuated so that a pneumatic signal is applied to one-way valves 11',12'whereby such valves are inoperable and permit bidirectional flow. Thismode of operation is illustrated schematically in FIG. 7. It will beseen that the configuration is a Mapleson A configuration correspondingto that illustrated in FIG. 1. Accordingly, in this configuration, theswitching valve 16 is maintained in its first position wherein the inletport thereof and thus the first flow passage 3 is connected to theventilator bag.

As shown in FIG. 13c, rotation of the cam shaft through 90° from theposition shown in FIG. 13d is effective to reverse the setting of theselector valve 2 such that, as shown in FIG. 14, the second flow passage4 is connected to the inlet port of the selector valve 16. The settingsof the valves 10 and 51 are, however, not altered. It will therefore beseen that this configuration is a Mapleson D configuration correspondingto that illustrated in FIG. 4. Accordingly, the switching valve 16 maybe set in either of its positions, depending on whether or notmechanical ventilation is required.

FIGS. 13a and 13b illustrate the configurations of the valve unit 50providing a circle system as shown in FIG. 15. In this configuration,which corresponds to that illustrated in FIG. 5, both the poppet valveassemblies of the selector valve 2 are open, so that the inlet to theswitching valve is connected to both of the flow passages 3, 4. Inaddition, the valve 51 is moved to a closed condition, such that apneumatic signal is no longer supplied to the one-way valves 11',12' andthese valves are operable accordingly. It will be seen that there aretwo angular positions of the cam shaft wherein the apparatus is set to acircle configuration. In the first shown in FIG. 13a, the valve 10 isclosed, so that gases passing through the second flow passage 4 areforced through the port 65 of the unit 50 and thus through the carbondioxide absorber. In the second condition shown in FIG. 13b, the valve10 is opened (the remaining valve settings remaining unaltered) so thatgases from the second flow passage may enter the unit 50 via the port 64and the carbon dioxide absorber is thus bypassed. As discussed above,the circle system is suitable for use either with the mechanicalventilator or with the ventilator bag, and accordingly the switchingvalve 16 may be set to either of its positions in this configuration.

It will be appreciated that with the valve unit shown in FIGS. 8 to 10,undesirable configurations are avoided, since the cams on the cam shaftare shaped such that only the four illustrated configurations may beselected. A further advantage of this embodiment is that the unit 50 isrelatively easy to dismantle for sterilization. In order to sterilizethose parts of the unit in contact with expelled breath of the patient,heat or chemical treatment is employed. It is therefore necessary toseparate the cam shaft and associated electrical components from theparts requiring sterilization. As shown in FIG. 9, this may readily beachieved by removing pins 90 which secure the cam shaft and associatedcomponents to the valve block 62.

We claim:
 1. Apparatus for controlling the flow of gas to or from apatient in an anaesthesia system, said apparatus comprising a reservoirbag, a selector valve means operable in a first condition to connectsaid bag to said first flow passage, in a second condition to connectsaid bag to said second flow passage, and in a third condition toconnect said bag to both of said first and second flow passages, saidfirst and second flow passages being arranged in parallel and each beingadapted to be connected at the end thereof remote from the selectorvalve means to a patient, said first passage comprising first one-wayvalve means selectively operable to prevent the back-flow of gas intosaid passage from the patient and communicating with an inlet for thesupply of fresh gas, and said second flow passage comprising secondone-way valve means selectively operable to prevent the flow of gas fromsaid second flow passage to the patient and a selectively bypassablecarbon dioxide absorbing means, there being a spill valve arranged torelease gases from the second flow passage, wherein means are providedto control said conditions of the selector valve means and the selectiveoperation of said first and second one-way valve means whereby desiredmodes of operation of the apparatus may be selected.
 2. Apparatus asclaimed in claim 1 further comprising a mechanical ventilator andswitching valve means actuable to connect either the ventilator or thereservoir bag to the selector valve means.
 3. Apparatus as claimed inclaim 2 wherein the apparatus is arranged to be supplied from a powersupply, and the switching valve is operable to connect the reservoir bagto the patient in the event of a break in the power supply to theapparatus.
 4. Apparatus as claimed in claim 2 wherein means are providedto prevent opening of the spill valve in synchronism with inhalationduring mechanical or manually controlled ventilation.
 5. Apparatus asclaimed in claim 4 wherein said spill valve includes a pressuresensitive closure member which is pressure communicable with a drive gasfor the mechanical ventilator during mechanical ventilation and with anoutlet from the reservoir bag during manually controlled ventilation. 6.Apparatus as claimed in claim 1 wherein the selector valve meanscomprises solenoid controlled closure members.
 7. Apparatus as claimedin claim 1 further comprising a rotable shaft having a plurality of cammembers, and wherein the selector valve means includes closure memberswhich are mechanically controlled by said cam members.
 8. Apparatus asclaimed in claim 1 wherein the one-way valve means comprise gravityoperated disc valves each having a valve closure disc.
 9. Apparatus asclaimed in claim 8 comprising selectively operable means for bypassingeach one-way valve means.
 10. Apparatus as claimed in claim 8 whereineach one-way valve means can be rendered inoperable by means of anelement selectively actuable to maintain the valve closure disc in anopen condition.
 11. Apparatus as claimed in claim 1 further comprisingmeans for sensing and monitoring the condition of the various valvemeans.